Many older persons have multiple, concurrent chronic illnesses, or "multimorbidity." At a time when scrutiny of quality of care is increasing, multimorbidity presents a unique challenge to the interpretation and application of evidence-based screening guidelines. For instance, screening colonoscopy is broadly conceived of as being effective and underused, with too few eligible patients undergoing the procedure. Yet the potential benefits of screening colonoscopy can vary substantially according to age and multimorbidity burden. Although it is intuitive that older, sicker patients have less to gain from screening colonoscopy, a framework based on population data is needed to determine how much older persons with varying burdens of multimorbidity could expect to benefit from undergoing screening colonoscopy. This framework would facilitate individualized decision-making at the bedside, inform the revision of screening guidelines, and allow focusing of quality of care initiatives on patients who are most likely to benefit from screening colonoscopy. Equally important is considering how targeting preventive strategies towards people with a lower burden of chronic illness could potentially adversely vulnerable populations, whose members carry a greater chronic illness burden. The objective of the proposed work is to further our understanding of how multimorbidity, sex, and age affect the risks and benefits of screening colonoscopy, with the intention of developing a framework that can be applied to other preventive health interventions. Population-based information about older cancer patients with colorectal cancer will be obtained from the linked Surveillance, Epidemiology, and End Results (SEER)-Medicare database. We will estimate (Aim 1), as a function of age, sex, and multimorbidity, the impact of screening colonoscopy on life expectancy, and the earliest time when the incremental benefits from screening colonoscopy exceed the incremental harms (i.e., the payoff time). After using a modeling approach to develop simple, transportable decision rules for determining which patients are likely or unlikely to benefit from a 1-time screening colonoscopy (Aim 2), Medicare claims data will be used to determine the population-level benefit that would accrue if colorectal cancer screening were more closely aligned with these decision rules (Aim 3). Finally, we will examine the effect of reallocating screening colonoscopies according to our calculated decision rules on vulnerable populations, including minority race and those with lower socioeconomic status, in terms of use of screening colonoscopy and over-all mortality (Aim 4). The resulting data will inform decision making at both the bedside and the health system level, by providing explicit decision rules as well as delineating the implications of implementing such rules for the over-all population, members of vulnerable populations, and individual patients.